This invention relates to a bag or other container having a valve which allows powdered or other particulate material to be fed into, but does not allow it to escape from, the bag.
Such valves have been formed, in the past, by a sleeve of paper, one end of which extends into the bag. When the bag is full, the pressure of the contents holds the sleeve closed. An example of such a bag is described in U.K. patent specification No. 1,245,720. These known valves unfortunately do not give a perfect seal, particularly against fine powders. The resulting loss of material is expensive and inconvenient and can present a health hazard to those who frequently handle the bags.
The problem of leakage can partially be overcome by cutting a series of slits at the inner end of the sleeve thereby forming a fringe. The strands of paper forming the fringe tend to be carried back through the sleeve with any reverse flow of the powder. The strands then form a blockage, preventing further reverse flow. While this technique does reduce leakage it presents another problem because the strands sometimes tear off and become mixed with the powder. This risk of pollution of the powder with strands of paper is unacceptable for many industrial processes.